Aminoplast resins are widely utilized as curing agents for polymer compositions. Aminoplast resins are typically prepared by reacting certain polyamines such as melamine or urea with an aldehyde such as formaldehyde to form alkylol group substituents on the amino nitrogens. The alkylol groups may be alkylated by reaction with an alcohol to form alkylalkoxy groups. The alkylalkoxy or alkylol groups are reactive with a variety of functional groups that can be appended to polymer backbones. Such functional groups include hydroxyl, acid (e.g., carboxyl), carbamate, amide, and ureido groups.
Curable polymer compositions such as those cured with aminoplast resins are usually made to achieve a desired specification with regard to the physical properties of the cured polymer. Often, it is desired to achieve a cured polymer that exhibits a high level of hardness without being brittle. This can be a particularly desirable objective in curable coating compositions, where hardness provides durability to the coating and flexibility provides smooth, contiguous coatings that do not crack or peel when subjected to stress, temperature variations, and the like.
One way to achieve a desired level of hardness is to control the crosslink density of the cured polymer composition. This can be accomplished by varying the level or amount of functional crosslinkable groups on the polymer or by varying the relative amounts of polymer and aminoplast in the curable composition. The control of the crosslink density is still somewhat limited, however, and often a tradeoff must be made between hardness and flexibility. The result of this is that desired levels of hardness and brittleness cannot be concurrently achieved. In certain applications, such as with the clearcoat layer of a color-plus-clear composite coating, this tradeoff manifests itself with the observation that when a coating has sufficient hardness to exhibit the desired durability, an undesirable level of brittleness causes a variety of problems such as environmental etch. Environmental etch manifests itself as spots or marks on or in the clear finish of the coating that often cannot be rubbed out. It is often difficult to predict the degree of resistance to environmental etch that a clearcoat will exhibit. Many coating compositions known for their durability and/or weatherability when used in exterior paints, such as high-solids enamels, do not provide the desired level of resistance to environmental etch when used as the clearcoat of a color-plus-clear composite coating. Thus, it would be desirable to provide further control over the crosslink density of aminoplast-cured polymer compositions.